Secondary-side control circuit, control method and flyback converter thereof

ABSTRACT

A method of controlling a secondary-side rectifier switch of a flyback converter, can include: detecting a slope parameter of a secondary-side detection voltage along a predetermined direction, where the secondary-side detection voltage is configured to represent a voltage across a secondary winding of the flyback converter; and controlling the secondary-side rectifier switch to turn on when the slope parameter is greater than a slope parameter threshold, and a relationship between the secondary-side detection voltage and the ON threshold meets a predetermined requirement.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201610379160.0, filed on Jun. 1, 2016, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of powerelectronics, and more particularly to secondary-side control circuitsand methods, and associated flyback converters.

BACKGROUND

Switch mode power supplies can efficiently convert electrical power froma source to a load, or to several different loads, with eachcorresponding to a different output. The main transistor of aswitching-mode supply can switch between on and off states at a givenoperating frequency, and voltage regulation can be achieved by varyingthe ratio of the on-to-off time of the main transistor. Switch modepower supplies may have relatively high power conversion efficiency, ascompared to other types of power converters. Switch mode power suppliesmay also be substantially smaller and lighter than a linear supply dueto the smaller transformer size and weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example flyback converterusing synchronous rectification technology.

FIG. 2 is a waveform diagram of example operation of the flybackconverter of FIG. 1 in a continuous conduction mode.

FIG. 3 is a waveform diagram of example operation of the flybackconverter of FIG. 1 in a discontinuous conduction mode.

FIG. 4 is a flow diagram of an example secondary-side control method fora flyback converter, in accordance with embodiments of the presentinvention.

FIG. 5 is a schematic block diagram of an example flyback converter, inaccordance with embodiments of the present invention.

FIG. 6 is a schematic block diagram of an example secondary-side controlcircuit, in accordance with embodiments of the present invention.

FIG. 7 is a schematic block diagram of another example secondary-sidecontrol circuit, in accordance with embodiments of the presentinvention.

FIG. 8 is a schematic block diagram of yet another examplesecondary-side control circuit, in accordance with embodiments of thepresent invention.

FIG. 9 is a waveform diagram of example operation of the secondary-sidecontrol circuit of FIG. 8, in accordance with embodiments of the presentinvention.

FIG. 10 is a flow diagram of example operation of the secondary-sidecontrol circuit of FIG. 8, in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Flyback converters have characteristics of relatively high conversionefficiency, and relatively low power losses. A typicalprimary-controlled flyback converter regulates an output voltage or anoutput current by controlling a main power switch at the primary-side ofthe transformer. In addition, at the secondary side of the flybackconverter, a rectifier switch may be used to replace a diode, in orderto significantly reduce rectifier power losses, and to improveconversion efficiency.

Referring now to FIG. 1, shown is a schematic block diagram of anexample flyback converter using synchronous rectification technology.Also referring to FIG. 2, which shows a waveform diagram of exampleoperation of the flyback converter of FIG. 1 in a continuous conductionmode. After main power switch M₁ is turned off based on control signalV_(G) _(_) _(PRI), drain voltage V_(D) _(_) _(PRI) of main power switchM₁ may rise. The energy stored in a transformer can start to betransferred to the secondary side, and a body diode of secondary-siderectifier switch M₂ may be firstly be turned on. Thus, voltage V_(D)_(_) _(SEC) at a common node of secondary-side rectifier switch M₂ and asecondary winding may be negative due to the voltage drop across thebody diode. When voltage V_(D) _(_) _(SEC) is lower than ON thresholdvalue V_(SRON) _(_) _(TH) (e.g., slightly lower than zero, such as about−50 mV), the secondary-side rectifier switch can be controlled to turnon according to control signal V_(G) _(_) _(SEC).

Referring now to FIG. 3, shown is a waveform diagram of exampleoperation of the flyback converter of FIG. 1 in a discontinuousconduction mode. However, when the flyback converter operates in adiscontinuous conduction mode, in each switching cycle, when the energystored in the transformer is fully transferred to the secondary side, avoltage across a primary winding may resonate due to the existence ofthe parasitic parameters prior to the main power switch at the primaryside being turned on. The voltage resonance may be transferred to thesecondary side through the transformer, and voltage V_(D) _(_) _(SEC)can accordingly resonate. If the resonance amplitude is relativelylarge, voltage V_(D) _(_) _(SEC) may be decayed to be lower than ONthreshold value V_(SRON) _(_) _(TH) during the resonance period, andthis condition may “mislead” secondary-side rectifier switch M₂ to beturned on.

When a main power switch at the primary side of a flyback converter isturned off, a voltage across a secondary winding may fall at arelatively high rate. When resonance occurs at the primary side, thevoltage across the secondary winding may fall at a relatively low rate.Voltage V_(D) _(_) _(SEC) can be used to represent the voltage acrossthe secondary winding before the secondary-side rectifier switch isturned on. In particular embodiments, the voltage drop across thesecondary winding that is caused by the turn off of the main powerswitch or the resonance that occurs after the energy stored in thetransformer is released can be determined. Thus, the secondary-siderectifier switch may be precisely controlled according to the fall rateof the voltage across the secondary winding.

In order to detect the fall slope of the voltage across the secondarywinding, secondary-side detection voltage V_(D) _(_) _(SEC) across thesecondary winding may be detected. Secondary-side detection voltageV_(D) _(_) _(SEC) can be detected during the change period fromthreshold value V_(D) _(_) _(HTH) to threshold value V_(D) _(_) _(HTH).When secondary-side detection voltage V_(D) _(_) _(SEC) is decayed to belower than threshold value V_(D) _(_) _(HTH), timing can begin, and whenV_(D) _(_) _(SEC) is decayed to be lower than V_(D) _(_) _(HTH), suchtiming can complete. For example, the change period can be denoted byΔT, so the equivalent fall slope of the voltage across the secondarywinding can be calculated as shown below in Equation (1).k=V _(D) _(_) _(HTH) −V _(D) _(_) _(LTH) /ΔT  (1)

It can be seen that fall period ΔT is in inverse proportion to the fallslope, so ΔT can be used as the basis of the secondary-side control. Thefall rate may be higher when ΔT is shorter, and may be lower when ΔT islonger. Generally, when the main power switch at the primary side isturned off, the fall slope of the voltage across the secondary windingis several times the fall slop in the resonance state. Correspondingly,the fall time of the voltage across the secondary winding in theresonance state may be several times the fall time when the main powerswitch at the primary side is turned off. By comparing the fall time(change period) against a time threshold, the voltage drop across thesecondary winding that is caused by resonance can be determined. Alsofor example, this can be determined by detecting fall amplitude ΔV inpredetermined period ΔTc when secondary-side detection voltage V_(D)_(_) _(SEC) starts falling to be lower than a given threshold value.Thus, the equivalent fall slope of the voltage across the secondarywinding can be calculated as shown below in Equation (2).k=ΔV/ΔTc  (2)

It can be seen that the fall amplitude is in direct proportion to thefall slope, and may be used as the basis of the secondary-side control.By comparing the fall time (change period) against an amplitudethreshold, the voltage drop across the secondary winding that is causedby resonance can be determined.

In one embodiment, a method of controlling a secondary-side rectifierswitch of a flyback converter, can include: (i) detecting a slopeparameter of a secondary-side detection voltage along a predetermineddirection, where the secondary-side detection voltage is configured torepresent a voltage across a secondary winding of the flyback converter;and (ii) controlling the secondary-side rectifier switch to turn on whenthe slope parameter is greater than a slope parameter threshold, and arelationship between the secondary-side detection voltage and the ONthreshold meets a predetermined requirement.

Referring now to FIG. 4, shown is a flow diagram of an examplesecondary-side control method for a flyback converter, in accordancewith embodiments of the present invention. In this example,“mis-conduction” or the turning on/off of the secondary-side rectifierswitch at times that may cause substantial residence, can besubstantially avoided. At S100, a slope parameter of the secondary-sidedetection voltage that changes along a predetermined direction can bedetected. The secondary-side detection voltage may represent a voltageacross the secondary winding of the flyback converter. Thesecondary-side detection voltage may be in in direct or inverseproportion to the voltage across the secondary winding. In one example,the secondary-side detection voltage utilizes drain voltage V_(D) _(_)_(SEC) of a secondary-side rectifier switch. Also, the slope parametercan be a change amplitude of the secondary-side detection voltage duringa predetermined time, and the threshold value of the slope parameter canbe the threshold value of the amplitude.

For example, the slope parameter may be the change time during which thesecondary-side detection voltage changes from a first threshold value toa second threshold value, and the threshold value of the slope parameteris the threshold value of the fall time. When the secondary-sidedetection voltage uses drain voltage V_(D) _(_) _(SEC) of thesecondary-side rectifier switch, the change time can be the actual falltime, and the first threshold value may be greater than the secondthreshold value. In other cases, a voltage detected at other positionsof the secondary-side circuit can be used as the secondary-sidedetection voltage. When the secondary-side detection voltage changes inan inverse direction with respect to the voltage across the secondarywinding, the change time may be the rise time, and the first thresholdvalue can be less than the second threshold value.

At S200, when the slope parameter is greater than the threshold value,and the relationship between the secondary-side detection voltage andthe conduction threshold value meets a predetermined requirement, thesecondary-side rectifier switch can be controlled to turn on. Forexample, the change threshold value that represents the threshold valueof the fall time is an amplification signal N times the previous validchange time signal, where N is greater than 1 but less than 5 (e.g.,greater than 2 and less than 3, such as N=2.5). The previous validchange time signal may be a change time signal which that is previouslydetected and used to turn on the secondary-side rectifier switch when inan active state, and the change time signal is used to represent thechange time.

For example, when the secondary-side detection voltage uses drainvoltage V_(D) _(_) _(SEC) of the secondary-side rectifier switch, theinitial value of the change threshold may be obtained according to thechange time signal corresponding to that the freewheeling time beinggreater than a reference time. For example, the freewheeling time is atime interval during which the drain voltage of the transistor is lessthan the freewheeling threshold. The following describes exemplarycircuit configurations that utilize control based on the detection ofthe change time.

In one embodiment, a control circuit for controlling a secondary-siderectifier switch of a flyback converter, can include: (i) a slopeparameter detection circuit configured to detect a slope parameter of asecondary-side detection voltage along a predetermined direction, and togenerate a slope detection signal that represents the slope parameter,where the secondary-side detection voltage is configured to represent avoltage across a secondary winding of the flyback converter; and (ii) aswitch control signal generator configured to generate a switchingcontrol signal to control the secondary-side rectifier switch to turn onwhen the slope detection signal is greater than a slope parameterthreshold, and a relationship between the secondary-side detectionvoltage and an ON threshold value meets a predetermined requirement.

Referring now to FIG. 5, shown is a schematic block diagram of anexample flyback converter, in accordance with embodiments of the presentinvention. In this particular example, the flyback converter can includepower stage circuit 1, primary-side control circuit 2, andsecondary-side control circuit 3. Power stage circuit 1 can includeprimary winding L₁, secondary winding L₂, main power switch M₁ coupledbetween the primary winding and ground, and secondary-side rectifierswitch M₂ coupled between secondary winding L₂ and ground. Both of mainpower switch M₁ and secondary-side rectifier switch M₂ can be anysuitable controllable electrical switch devices (e.g., metal oxide fieldeffect transistors [MOSFET], bipolar junction transistors [BJT], etc.).Primary-side control circuit 2 can control the switching operation ofmain power switch M₁. Primary-side control circuit 2 may be implementedby any suitable circuit structure. Secondary-side rectifier switch 3 cancontrol the switching operation of secondary-side rectifier switch M₂,in order to realize synchronous rectification, and to output a stablevoltage or current to a load.

Secondary-side control circuit 3 can include change time detectioncircuit 31 and switch control signal generator 32. In this particularexample, drain voltage V_(D) _(_) _(SEC) of secondary-side rectifierswitch M₂ is taken as an example to represent the secondary-sidedetection voltage, because drain voltage V_(D) _(_) _(SEC) can representthe voltage across the secondary winding when secondary-side rectifierswitch M₂ is turned off. Change time detection circuit 31 can be used toobtain the change time signal V_(ΔT). Change time signal V_(ΔT) mayrepresent change time ΔT during which secondary-side detection voltageV_(D) _(_) _(SEC) changes from threshold value V_(D) _(_) _(HTH) tothreshold value V_(D) _(_) _(LTH). It can be seen that change timesignal V_(ΔT) is proportional to change time ΔT.

When change time ΔT represented by change time signal V_(ΔT) is lessthan fall time threshold ΔT_TH corresponding to change threshold V_(ΔT)_(_) _(TH), and the relationship of secondary-side detection voltageV_(D) _(_) _(SEC) and ON threshold value V_(SRON) _(_) _(TH) meets apredetermined requirement, switch control signal generator 32 canactivate a switching control signal to turn on secondary-side rectifierswitch M₂. In this particular example, the predetermined requirement forthe relationship between secondary-side detection voltage V_(D) _(_)_(SEC) and ON threshold value V_(SRON) _(_) _(TH) can indicate thatsecondary-side detection voltage V_(D) _(_) _(SEC) is decayed to belower than ON threshold value V_(SRON) _(_) _(TH). For example, changetime signal V_(ΔT) is in direct proportion to change time ΔT. Thus, therelationship between change time ΔT and fall time threshold ΔT_TH can bedetermined by determining if change voltage signal V_(ΔT) is less thanchange threshold value V_(ΔT) _(_) _(TH).

For example, switch control signal generator 32 can generate theswitching control signal to turn off secondary-side rectifier switch M₂according to a reset pulse signal generated by OFF control circuit 33.OFF control circuit 33 can be implemented by any suitable circuitry. Itcan be seen that, when the voltage change amplitude in a predeterminedtime is used as the slope parameter, the change time detection circuitmay be replaced by a change amplitude detection circuit, and the switchcontrol signal generator can compare the change amplitude and theamplitude threshold value, and may trigger the secondary-side rectifierswitch to turn on when the change amplitude is greater than theamplitude threshold value, and the relationship between secondary-sidedetection voltage V_(D) _(_) _(SEC) and ON threshold value V_(SRON) _(_)_(TH) meets a predetermined requirement.

Thus, the slope parameter of the secondary-side detection voltage alonga predetermined direction can be detected by the slope parameterdetection circuit (e.g., change time detection circuit or changeamplitude detection circuit), in order to obtain the slope detectionsignal to represent the slope parameter. Then, the switch control signalgenerator may generate the switching control signal in order to turn onthe secondary-side rectifier switch in a case when the slope parameterrepresented by the slope detection signal is greater than the slopeparameter threshold, and the relationship between the secondary-sidedetection voltage and the ON threshold value meets the predeterminedrequirement. In such a manner, whether the voltage drop of the secondarywinding is caused by the turn off of the main power switch at theprimary side can effectively be distinguished from the voltage drop ofthe secondary winding being due to the parasitic parameters, in order toavoid mis-conduction of the secondary-side rectifier switch.

For example, change time detection circuit 31 can include a timing andreset circuit and a signal conversion circuit. The timing and resetcircuit can activate control signal V1 when secondary-side detectionvoltage V_(D) _(_) _(SEC) changes from threshold value V_(D) _(_) _(HTH)to threshold value V_(D) _(_) _(LTH), and may activate control signal V2when the secondary-side detection voltage is less than threshold valueV_(D) _(_) _(LTH) but greater than threshold value V_(D) _(_) _(HTH).Control signal V1 can control the signal conversion circuit for timingthe active duration of control signal V1, and the valid duration is thechange time interval. Control signal V2 may reset the signal conversioncircuit whereby change time signal V_(ΔT) is reset to the initial value(e.g., zero). After ending the timing and before reset, the signalconversion circuit may maintain change time signal V_(ΔT) obtained bytiming.

The time point for resetting the signal conversion circuit may have aplurality of choices. For example, the reset can occur when thesecondary-side detection voltage is less than threshold value V_(D) _(_)_(LTH) (i.e., the timing ends) for a predetermined delay. In anotherexample, the reset can occur when secondary-side detection voltage V_(D)_(_) _(SEC) rises to be greater than threshold value V_(D) _(_) _(LTH)again, or rises to be greater than threshold value V_(D) _(_) _(HTH).The purpose of reset is to guarantee the accuracy of subsequent timing,and to prevent the next timing result from being affected by theprevious timing result. Thus, reset can be carried out when thesecondary-side detection voltage V_(D) _(_) _(SEC) falls to thresholdvalue V_(D) _(_) _(HTH) again and before the next timing is started. Thesignal conversion circuit can generate a voltage signal that isproportional to the valid time of control signal V1 as change timesignal V_(ΔT), and may reset the voltage signal according to the validpulses of control signal V2.

Referring now to FIG. 6, shown is a schematic block diagram of anexample secondary-side control circuit, in accordance with embodimentsof the present invention. In this example, the timing and reset circuitof change time detection circuit 31 can include comparators CMP3-CMP5,pulse generators OS1-OS3, and RS flip-flop RS1. Comparator CMP3 cancompare secondary-side detection voltage V_(D) _(_) _(SEC) againstthreshold value V_(D) _(_) _(HTH), and may generate a high level whensecondary-side detection voltage V_(D) _(_) _(SEC) is lower thanthreshold value V_(D) _(_) _(HTH). Comparator CMP4 can compare thresholdvalue V_(D) _(_) _(LTH) against secondary-side detection voltage V_(D)_(_) _(SEC), and may output a high level when secondary-side detectionvoltage V_(D) _(_) _(SEC) is lower than threshold value V_(D) _(_)_(LTH). Pulse generator OS1 can connect to an output terminal ofcomparator CMP3, and may provide triggering pulses in respond to therising edges (triggering edges) of the comparison signal. Pulsegenerator OS2 can connect to an output terminal of comparator CMP4, andmay provide triggering pulses in response to the rising edges(triggering edges) of the comparison signal. RS trigger RS1 may have aset terminal coupled to pulse generator OS1, a reset terminal coupled topulse generator OS2, and an output terminal for providing control signalV1.

When secondary-side detection voltage V_(D) _(_) _(SEC) falls to belower than threshold value V_(D) _(_) _(HTH), comparator CMP3 can outputa comparison signal with a rising edge. Pulse generator OS1 can output atriggering pulse in response to the rising edge of the comparisonsignal, so as to set RS flip-flop RS1, and control signal V1 may beactivated (e.g., at a high level). When secondary-side detection voltageV_(D) _(_) _(SEC) continuously falls to be lower than threshold valueV_(D) _(_) _(LTH), comparator CMP4 can output a comparison signal with arising edge. Pulse generator OS2 may generate a triggering pulse inresponse to the rising edge of the comparison signal, so as to reset RSflip-flop RS1, and control signal V1 may be deactivated (e.g., go low).In this way, the change time during which secondary-side detectionvoltage V_(D) _(_) _(SEC) changes from threshold value V_(D) _(_) _(HTH)to threshold value V_(D) _(_) _(LTH) can be timed, and control signal V1can effectively characterize the change time interval.

In addition, comparator CMP5 can compare threshold value V_(D) _(_)_(LTH) against secondary-side detection voltage V_(D) _(_) _(SEC), andmay output a high level when secondary-side detection voltage V_(D) _(_)_(SEC) is lower than threshold value V_(D) _(_) _(LTH). Pulse generatorOS3 can connect to comparator CMP5, and may output control signal V2 inthe form of pulses in response to the rising edge of the output signalof comparator CMP5 (e.g., in response to the case that secondary-sidedetection voltage V_(D) _(_) _(SEC) rises to be greater than thresholdvalue V_(D) _(_) _(LTH)). In this way, the signal conversion circuit maybe reset when secondary-side detection voltage V_(D) _(_) _(SEC) risesto be greater than threshold value V_(D) _(_) _(LTH). One skilled in theart will recognize that pulse generator OS2 can have different inputswhen the reset timings are different, and the triggering edges of pulsegenerator OS2 can be different on the basis of different valid/activelevels.

The signal conversion circuit can include current source A1, controlswitch K1, capacitor C1, control switch K2, and voltage controlledcurrent source U1. Current source A1 and control switch K1 can connectin series between a supply terminal and an intermediate terminal “m.”Control switch K1 may be controlled by control signal V1. Capacitor C1and control switch K2 can connect in parallel between intermediateterminal “m” and ground. Control switch K2 may be controlled by controlsignal V2. Voltage controlled current source U may generate an outputvoltage as change time signal V_(ΔT), and the output voltage can beproportional to a voltage across capacitor C1. When control signal V1 isactive, control switch K1 may be turned on, control switch K2 can beturned off, and current source A1 may charge capacitor C1. The voltageacross capacitor C1 may linearly rise along with the duration time inwhich control signal V1 is active. When control signal V1 isdeactivated, control switch K1 can be turned off, control switch K2 maybe turned off, and the voltage across capacitor C1 remains unchanged.When control signal V2 is activated, control signal K2 can be turned onsuch that capacitor C1 is discharged, and the voltage across capacitorC1 can be reset to zero. A voltage at intermediate terminal “m” (e.g.,the voltage across capacitor C1) may be replicated as change time signalVΔT at the output terminal steadily by voltage controlled current sourceU1.

As also shown in FIG. 5, switch control signal generator 32 can includecomparator CMP1, comparator CMP2, and logic circuit LG. Comparator CMP1can compare change threshold value V_(ΔT) _(_) _(TH) against change timesignal V_(ΔT), and may generate comparison signal V_(C1). ComparatorCMP2 can compare ON threshold V_(SRON) _(_) _(TH) against secondary-sidedetection voltage V_(D) _(_) _(SEC), and may generate comparison signalV_(C2). Logic circuit LG may provide the switching control signal toturn on the secondary-side rectifier switch in a case that comparisonsignal V_(C1) indicates change time signal V_(ΔT) is less than changethreshold value V_(ΔT) _(_) _(TH), and comparison signal V_(C2)indicates secondary-side detection voltage V_(D) _(_) _(SEC) is lessthan ON threshold value V_(SRON) _(_) _(TH). For example, changethreshold value V_(ΔT) _(_) _(TH) may be a predetermined voltage valueprovided externally or by a voltage source.

Logic circuit LG can include pulse generator OS0, AND-gate AND1, and RSflip-flop RS0. Pulse generator OS0 can connect to comparator CMP2, andmay generate a pulse with a predetermined width in response to therising edge of comparison signal V_(C2) (e.g., when secondary-sidedetection voltage V_(D) _(_) _(SEC) is less than ON threshold valueV_(SRON) _(_) _(TH)). Pulse and comparison signal V_(C1) generated bycomparator CMP1 may be provided to AND-gate AND1. The output terminal ofAND-gate AND1 can connect to the set terminal of RS flip-flop RS0.Because comparison signal V_(C1) can represent the relationship betweenchange time signal V_(ΔT) and change threshold value V_(ΔT) _(_) _(TH),when the voltage across the secondary winding falls due to the turn offof main power switch M1 at the primary side, ΔT<ΔT_TH, and change timesignal V_(ΔT) is less than the change threshold value V_(ΔT) _(_) _(TH),comparison signal V_(C1) may be at a high level.

When secondary-side detection voltage V_(D) _(_) _(SEC) is less than ONthreshold value V_(SRON) _(_) _(TH), pulse generator OS0 can generateone pulse. AND-gate AND1 may output a set pulse when both of comparisonsignal V_(C1) and the pulse are active, so as to set RS flip-flop RS0,and may generate switching control signal V_(G) _(_) _(SEC) to turn onsecondary-side rectifier switch M2. When the voltage across thesecondary winding falls due to the resonance at the primary side,ΔT>ΔT_TH, thus change time signal V_(ΔT) is greater than changethreshold value V_(ΔT) _(_) _(TH), comparison signal V_(C1) may be low.Secondary-side detection voltage V_(D) _(_) _(SEC) can fall to be lessthan ON threshold value V_(SRON) _(_) _(TH) due to the relatively largeresonance amplitude at the secondary side. RS flip-flop RS0 may notoperate since comparison signal V_(C1) can be maintained at the lowlevel, and AND-gate AND1 can output a low level, in order to avoidmis-conduction of switching control signal V_(G) _(_) _(SEC).

Those skilled in the art will recognize that the connection relationshipbetween the logic circuit and the comparator in FIGS. 5 and 6 is onlyone exemplary arrangement, and various circuit structures employingdifferent logic circuit types, valid signal levels, and connectionrelationships between the logic circuits and comparators, canalternatively be used in certain embodiments. In addition, OFF controlcircuit 33 can provide a reset pulse to reset terminal R of RS flip-flopRS0 in logic circuit LG, such that switching control signal V_(G) _(_)_(SEC) is switched to indicate OFF.

In the present example, by using the characteristic that the voltageacross the secondary winding fluctuates slowly during the resonanceperiod, the change time signal indicating the change time of thesecondary-side detection voltage can be compared against the changethreshold value. Only when the change time of the secondary-sidedetection voltage is relatively small (e.g., the change rate isrelatively high and the slope is relatively large), the secondary-siderectifier switch may be allowed to turn on when the secondary-sidedetection voltage is less than the ON threshold value, therebydistinguishing whether the voltage drop of the secondary winding is dueto the turn off of the main power switch at the primary side or due toparasitic parameters, in order to substantially avoid mis-conduction ofthe secondary-side rectifier switch.

Referring now to FIG. 7, shown is a schematic block diagram of anotherexample secondary-side control circuit, in accordance with embodimentsof the present invention. In this example, the secondary-side controlcircuit can also include latch circuit 34 and gain circuit 35 inaddition to change time detection circuit 31 and switch control signalgenerator 32. Here, change threshold value V_(ΔT) _(_) _(TH) indicatingthe fall time threshold value can be latched and updated dynamically. Inthis particular example, the change threshold value is an amplificationsignal N times the previous valid change time signal, and N is greaterthan 1 but less than 5 (e.g., N is greater than 2 and less than 3, suchas N=2.5). The previous valid change time signal is a change time signalpreviously detected and used to turn on the secondary-side rectifierswitch when active.

Latch circuit 34 can be coupled to change time detection circuit 31 forlatching the previous valid change time signal. For example, latchcircuit 34 can include capacitor C2 and control switch K3, and capacitorC2 can connect between an output terminal of latch circuit 34 andground. Control switch K3 can connect between terminal “a” of latchcircuit 34 and the output terminal of change time detection circuit 31,and can be controlled to turn on for a predetermined time whensecondary-side rectifier switch M2 is switched from OFF to ON, such thatthe change time signal is transferred to capacitor C2 for storage. Thecontrol signal of control switch K3 can be a rising edge trigger signalof switching control signal V_(G) _(_) _(SEC) of secondary-siderectifier switch M2. The control signal can be generated by a pulsetrigger, or may be obtained from the signal at set terminal S of RSflip-flop RS1 in logic circuit LG.

When RS flip-flop RS1 is set and switching control signal V_(G) _(_)_(SEC) is switched to a high level, control switch K3 can be controlledto turn on for a predetermined time, such that the voltage acrosscapacitor C2 is updated to new change time signal V_(ΔT). Becauseswitching control signal V_(G) _(_) _(SEC) is switched to a high level,latched change time signal V_(ΔT) is a previous valid change time signalfor the next voltage drop of the secondary winding. Gain circuit 35 canconnect to latch circuit 34 for providing an amplification signal Ntimes the latched signal of the latch circuit. That is to say, thelatched signal of the latch circuit may be amplified by N times, and thelatched signal can be a voltage signal. For example, gain circuit 35 canbe implemented by voltage controlled voltage source U2. Change timesignal V_(ΔT) may be amplified by several times through gain circuit 35,and maintained in a reasonable range.

Thus in particular embodiments, the change threshold value mayself-regulate with the change of the circuit, which makes thesecondary-side control circuit more accurate and timely than asecondary-side control circuit with a fixed threshold value. Also, sincethe latched signal is updated in real time, it is possible to preventthe change threshold value from being inaccurate due to the charge lossof the capacitor. When the flyback converter is powered up, the changethreshold value typically needs an initial value for turning onsecondary-side rectifier switch M2 for the first time period in thisexample, the initial value can be set according to the change timesignal that is obtained by a latch operation after the flyback converteris powered up for predetermined time T_(SR) _(_) _(BLANK). Before theflyback converter is powered up, the secondary-side control circuit mayonly receive the change time signal but not latch it. In this particularexample, time T_(SR) _(_) _(BLANK) can be set as a fixed value, and maybe generated by a delay circuit.

Referring now to FIG. 8, shown is a schematic block diagram of yetanother example secondary-side control circuit, in accordance withembodiments of the present invention. In this particular example, thesecondary-side control circuit can also include latch control circuit 36in addition to change time detection circuit 31, switch control signalgenerator 32, latch circuit 34, and gain circuit 35. Latch controlcircuit 36 can control the latch timing of latch circuit 34, in order tofurther improve the control accuracy. For example, latch control circuit36 can control the latch circuit to operate when the freewheelingduration time is greater than a reference time for the first time afterthe flyback converter is powered up. Freewheeling duration time T_(DIS)is a duration time in which the drain voltage of the transistor is lessthan a freewheeling threshold value. Latch control circuit 36 can alsocontrol latch circuit 34 to operate each time when the secondary-siderectifier switch switches from OFF to ON.

In this particular example, latch control circuit 36 can control latchcircuit 34 to operate according to the signal at set terminal S of RSflip-flop RS1 in logic circuit LG. For the first latch operation afterthe flyback converter is powered up, latch control circuit 36 maydetermine if the previous fall was caused by the turn off ofprimary-side main power switch M₁ according to the duration time inwhich secondary-side detection voltage V_(D) _(_) _(SEC) is less thanthe freewheeling threshold value. The voltage drop of the secondarywinding can be caused by the resonance at the primary side, and drainvoltage V_(D) _(_) _(SEC) of secondary-side rectifier switch M₂ may beless than zero volts for a very short time and can be oscillated back tobe higher than zero volts relatively quickly. If the voltage drop of thesecondary winding is caused by the turn off of the main power switch atthe primary side, drain voltage V_(D) _(_) _(SEC) of secondary-siderectifier switch M₂ may be less than zero for a relatively long time. Inthis way, the latch timing of the latch circuit can be controlled.

For example, latch control circuit 36 can include comparator CMP6,OR-gate OR1, control switch K4, current source A2, capacitor C3,capacitor CMP7, pulse generator OS4, OR-gate OR2, and RS flip-flop RS2.Comparator CMP6 can compare secondary-side detection voltage V_(D) _(_)_(SEC) and freewheeling threshold value V_(DIS) _(_) _(TH), and maygenerate a comparison signal. In this particular example, freewheelingthreshold value V_(DIS) _(_) _(TH) can be zero. OR-gate OR1 can receivethe comparison signal and status signal V_(ST), and may generate controlsignal V3. Status signal V_(ST) can represent if the initial value ofthe change threshold value is latched or not. Control switch K4, currentsource A2, and capacitor C3 can connect in parallel between timingoutput terminal “b” and ground. Comparator CMP7 can compare a voltage attiming output terminal “b” that represents freewheeling time T_(D1)against voltage V_(SR) _(_) _(REF) that represents reference time T_(SR)_(_) _(REF), and may provide another comparison signal. Pulse generatorOS2 can generate a pulse signal in response to the rising edge orfalling edge of this comparison signal. The rising edge or falling edgemay be determined by the connection relationship of input signals ofcomparator CMP6. OR-gate OR2 may have an input terminal coupled to pulsegenerator OS2, and an input terminal for receiving a set signal of theswitching control signal.

Referring now to FIG. 9, shown is a waveform diagram of exampleoperation of the secondary-side control circuit of FIG. 8, in accordancewith embodiments of the present invention. After the flyback converteris powered up, the transformer is energized, and a transformer currentI_(T) rises and falls with the turn on and off of the main power switchat the primary side. In this example, the curve diagram of thetransformer current can include a primary-side current during the ONstate of the main power switch in ON state, and a convertedsecondary-side current during the OFF state of the main power switch.The initial state of status signal V_(ST) generated by RS flip-flop RS2is low, and comparator CMP6 can output a low level when secondary-sidedetection voltage V_(D) _(_) _(SEC) is less than freewheeling thresholdvalue V_(DIS) _(_) _(TH). Thus, OR-gate OR2 can continue to output a lowlevel during the period when secondary-side detection voltage V_(D) _(_)_(SEC) is less than freewheeling threshold value V_(DIS) _(_) _(TH), andcontrol switch K4 remains off. Current source A2 can continue chargingcapacitor C3, such that the voltage at timing output terminal “b” riseswith the duration time in which secondary-side detection voltage V_(D)_(_) _(SEC) is less than freewheeling threshold value V_(DIS) _(_)_(TH).

When secondary-side detection voltage V_(D) _(_) _(SEC) is greater thanfreewheeling threshold value V_(DIS) _(_) _(TH), comparator CMP6 canoutput a high level, OR-gate OR2 may output a high level, control switchK4 can be turned on, and capacitor C3 may be reset. After the flybackconverter is powered up, the latch control circuit can repeat the aboveoperation until the time duration in which secondary-side detectionvoltage V_(DIS) _(_) _(SEC) s less than freewheeling threshold valueV_(DIS) _(_) _(TH) (e.g., zero volts) is large enough to make thevoltage at timing output terminal “b” greater than voltage V_(SR) _(_)_(REF) that indicates reference time T_(SR) _(_) _(REF). Comparator CMP7can output high level. Pulse generator OS4 may output pulse V4 inresponse to the rising edge, such that the high level pulse generated byOR-gate OR3 can control latch circuit 34 to latch for the first timeafter the flyback converter is powered up, thereby setting the initialvalue of the change threshold value.

Referring also to FIG. 8 in conjunction with FIG. 9, the set terminal ofRS flip-flop RS2 can connect to the output of pulse generator OS4, thereset terminal can connect to ground, and the output terminal maygenerate status signal V_(ST). After pulse generator OS4 outputs a pulsefor the first time, RS flip-flop RS2 can be set, and status signalV_(ST) may transition from low to high in order to indicate that thefirst latch operation is complete. Since status signal V_(ST) may remainhigh, control switch K4 can remain on, and the output of pulse generatorOS2 can remain low. The output of OR-gate OR2 may be related to theother input (e.g., the set signal of the switching control signal).Thus, after the first latch operation is completed, latch controlcircuit 36 can control latch circuit 34 to operate each time whensecondary-side rectifier switch M2 is turned on, in order to provide thechange threshold value for determining the next conduction.

Referring now to FIG. 10, shown is a flow diagram of example operationof the secondary-side control circuit of FIG. 8, in accordance withembodiments of the present invention. After the flyback converter ispowered up, at S1000, secondary-side control circuit 3 may prohibitsecondary-side rectifier switch M2 from turning on, and can detect thechange time ΔT at the falling edge of the secondary-side detectionvoltage V_(D) _(_) _(SEC). Secondary-side control circuit 3 can alsodetect duration time T_(DIS) in which secondary-side detection voltageV_(D) _(_) _(SEC) is less than the freewheeling threshold value afterthe falling edge. During this time duration, the rectification at thesecondary side may be realized by a body diode of the secondary-siderectifier switch. When the freewheeling at the secondary side isstarted, the body diode can be turned on, such that drain voltage V_(D)_(_) _(SEC) goes negative. At S2000, it can be determined if durationtime T_(DIS) is greater than reference time T_(SR) _(_) _(REF). If yes,the flow can proceed to S1000, and if not the flow can proceed to S3000.

If secondary-side detection voltage V_(D) _(_) _(SEC) goes negative andthe duration time exceeds reference time T_(SR) _(_) _(REF), the latchcontrol circuit may generate a pulse signal when reference time T_(SR)_(_) _(REF) has elapsed, in order to latch detected change time signalV_(ΔT) and to generate the change threshold signal. Otherwise, ifsecondary-side detection voltage V_(D) _(_) _(SEC) may go negative andthe duration time can be relatively short to be less than reference timeT_(SR) _(_) _(REF), and the control circuit may neglect the previouslydetected change time signal.

At S3000, change time ΔT can be latched in the current period toΔT_(REF), and secondary-side rectifier switch M2 can be controlled toturn on. At S4000, change time ΔT can be latched at the falling edge ofvoltage V_(D) _(_) _(SEC). At S5000, it can be determined if change timeΔT is less than fall time threshold value ΔT_TH=N*ΔT_(REF). If yes, theflow can proceed to S6000, and if not the flow can proceed to S7000. AtS6000, when the secondary-side detection voltage V_(D) _(_) _(SEC) isless than ON threshold value V_(SRON) _(_) _(TH), the flow can returnback to S3000. At S7000, when secondary-side detection voltage V_(D)_(_) _(SEC) is less than ON threshold value V_(SRON) _(_) _(TH),secondary-side rectifier switch M₂ can be prohibited from turning on,and the flow can return back to S4000.

In this way, after the flyback converter is powered up, rectification bythe body diode of the secondary-side rectifier switch can occur, and itcan be determined if the currently detected falling edge is caused bythe turn off of the main power switch at the primary side and if thecircuit enters into the steady state on the basis of the duration timeof the secondary-side detection voltage. The first latch operation maybe carried out only when the circuit enters into the steady state andthe currently detected falling edge is caused by the turn off of themain power switch at the primary side, in order to obtain the initialvalue of the change threshold value. This can improve the controlaccuracy of the circuit, and may guarantee that the circuit operatessteadily after being powered up.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to particularuse(s) contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. A control circuit for controlling asecondary-side rectifier switch of a flyback converter, the controlcircuit comprising: a) a change time detection circuit configured todetect a change time during which a secondary-side detection voltagechanges from a first to a second threshold value along a predetermineddirection, and to generate a change time signal that represents saidchange time, wherein said secondary-side detection voltage is configuredto represent a voltage across a secondary winding of said flybackconverter; and b) a switch control signal generator configured togenerate a switching control signal to control said secondary-siderectifier switch to turn on when said change time signal is less than afall time threshold represented by a change threshold, and arelationship between said secondary-side detection voltage and an ONthreshold value meets a predetermined requirement.
 2. The controlcircuit of claim 1, wherein said switch control signal generatorcomprises: a) a first comparator configured to compare a changethreshold value against said change time signal, and to generate a firstcomparison signal; b) a second comparator configured to compare an ONthreshold value against said secondary-side detection voltage, and togenerate a second comparison signal; and c) a logic circuit configuredto generate said switching control signal to control said secondary-siderectifier switch to turn on when said first comparison signal indicatessaid change time signal is less than said change threshold value, andsaid second comparison signal indicates said relationship between saidsecondary-side detection voltage and said ON threshold value meets saidpredetermined requirement.
 3. The control circuit of claim 1, wherein:a) said change threshold value is an amplification signal N times aprevious valid change time signal, and N is greater than 1 and less than5; and b) said previous valid change time signal is a change time signalwhich is previously detected and is used to turn on said secondary-siderectifier switch when activated.
 4. The control circuit of claim 3,wherein said secondary-side control circuit further comprises: a) alatch circuit coupled to said change time detection circuit, and beingconfigured to latch said previous valid change time signal; and b) again circuit coupled to said latch circuit, and being configured togenerate said amplification signal.
 5. The control circuit of claim 4,wherein said latch circuit comprises: a) a second capacitor coupledbetween an output terminal of said latch circuit and ground; and b) athird control switch coupled between said output terminal of said latchcircuit and an output terminal of said change time detection circuit,and being configured to control when said secondary-side rectifierswitch is switched from OFF to ON.
 6. The control circuit of claim 4,wherein said secondary-side control circuit further comprises: a) alatch control circuit configured to control said latch circuit tooperate when said freewheeling duration time is detected to be greaterthan a reference time for the first time after being powered up, and tocontrol said latch circuit to operate each time when said secondary-siderectifier switch is switched from OFF to ON; and b) said freewheelingduration time is a time duration in which said secondary-side detectionvoltage is less than a freewheeling threshold value.
 7. The controlcircuit of claim 6, wherein said latch control circuit comprises: a) athird comparator configured to compare said secondary-side detectionvoltage against said freewheeling threshold value, and to generate athird comparison signal; b) a first OR-gate configured to receive saidthird comparison signal and a status signal, and to generate a thirdcontrol signal, wherein said status signal represents whether an initialvalue of said change threshold value is latched; c) a fourth controlswitch, a current source, and a third capacitor coupled in parallelbetween a timing output terminal and ground; d) a fourth comparatorconfigured to compare a voltage at said timing output terminal and avoltage representing said reference time, and to generate a fourthcomparison signal; e) a fourth pulse generator configured to generate apulse signal in response to a rising edge or a falling edge of saidfourth comparison signal; f) a second OR-gate having a first inputterminal coupled to said fourth pulse generator, and a second inputterminal coupled to a set signal of said switching control signal; andg) a second RS flip-flop having a set terminal coupled to said fourthpulse generator, a reset terminal coupled to ground, and an outputterminal configured to generate said status signal.
 8. The controlcircuit of claim 1, wherein: a) said secondary-side rectifier switch isa transistor, and said secondary-side detection voltage is a drainvoltage of said transistor; b) said change time detection circuitcomprises a timing and reset circuit configured to activate a firstcontrol signal when said secondary-side detection voltage falls fromsaid first to said second threshold value, and to activate a secondcontrol signal before said secondary-side detection voltage falls tosaid first threshold value again; and c) said change time detectioncircuit comprises a signal conversion circuit configured to generate avoltage signal proportional to an active time of said first controlsignal as said change time signal, and to reset said voltage signal whensaid second control signal is activated.
 9. The control circuit of claim8, wherein said timing and reset circuit is configured to activate saidsecond control signal after a predetermined delay when at least one of:said secondary-side detection voltage is less than said second thresholdvalue, said secondary-side detection voltage increases to be greaterthan said second threshold value, and said secondary-side detectionvoltage increases to be greater than said first threshold value.
 10. Thecontrol circuit of claim 9, wherein said timing and reset circuitcomprises: a) a third comparator configured to generate a thirdcomparison signal with triggering edges when said secondary-sidedetection voltage falls to be less than said first threshold value; b) afirst pulse generator configured to generate a first triggering pulseaccording to said triggering edges of said third comparison signal; c) afourth comparator configured to generate a fourth comparison signal withtriggering edges when said secondary-side detection voltage falls to beless than said second threshold value; d) a second pulse generatorconfigured to generate a second triggering pulse according to saidtriggering edges of said fourth comparison signal; e) a first RSflip-flop having a set terminal coupled to said first pulse generator, areset terminal coupled to said second pulse generator, and an outputterminal configured to generate said first control signal; f) a fifthcomparator configured to generate a fifth comparison signal withtriggering edges when said secondary-side detection voltage rises to begreater than said second threshold value; and g) a third pulse generatorconfigured to activate said second control signal according to saidtriggering edges of said fifth comparison signal.
 11. The controlcircuit of claim 8, wherein said signal conversion circuit comprises: a)a first current source and a first control switch coupled in seriesbetween a supply terminal and an intermediate terminal, wherein saidfirst control switch is controlled by said first control signal; b) afirst capacitor and a second control switch coupled in parallel betweensaid intermediate terminal and ground, wherein said second controlswitch is controlled by said second control signal; and c) a voltagecontrolled current source configured to generate an output voltageproportional to a voltage across said first capacitor.
 12. A powerconverter comprising the secondary-side control circuit of claim 1, andfurther comprising: a) a power stage circuit in a flyback configuration,said power stage circuit having a main power switch coupled between aprimary winding of a transformer and a ground terminal; b) saidsecondary-side rectifier switch being coupled between a secondarywinding of said transformer and said ground terminal; and c) aprimary-side control circuit for controlling said main power switch. 13.A method of controlling a secondary-side rectifier switch of a flybackconverter, the method comprising: a) detecting a change time duringwhich a secondary-side detection voltage changes from a first to asecond threshold value along a predetermined direction, wherein saidsecondary-side detection voltage is configured to represent a voltageacross a secondary winding of said flyback converter; and b) controllingsaid secondary-side rectifier switch to turn on when said change time isless than a fall time threshold, and a relationship between saidsecondary-side detection voltage and said ON threshold meets apredetermined requirement.
 14. The method of claim 13, wherein a changethreshold value for representing said fall time threshold is apredetermined fixed value.
 15. The method of claim 13, wherein a changethreshold value for representing said fall time threshold is anamplification signal N times a previous valid change time signal, said Nis greater than 1 and less than 5, said previous valid change timesignal is a change time signal that was previously detected and is usedto turn on said secondary-side rectifier switch when activated, and saidchange time signal is configured to represent said change time.
 16. Themethod of claim 15, wherein: a) said secondary-side rectifier switch isa transistor, and said secondary-side detection voltage is a drainvoltage of said transistor; b) an initial value of said change thresholdvalue is obtained according to a change time signal corresponding to afreewheeling duration time being greater than a reference time; and c)said freewheeling duration time is a time duration during which a drainvoltage of said transistor is less than a freewheeling threshold value.